Find the truth table for the circuit shown. Explain the working principle for all the inputs, briefly. Explain why D1 is used in the network.

  see below for the truth table

Explanation:

Truth Table

As we will see from the description of operation, any input low causes the output to be high. This is the logic of a NAND gate. The truth table is attached.

Working Principle

Pulling any of A, B, or C low will saturate transistor Q1, depriving Q2 of any base current, cutting it off. Then Q5 is also deprived of base current and is cut off. Meanwhile, the current through R2 supplies base current to Q4, allowing it to pull the output high.

If all of A, B, and C are high (or open), base current is supplied to Q2 through the base-collector junction of Q1. Then Q2 saturates, supplying base current to Q3. Diode D1 ensures that the voltage across Q2 will be insufficient to supply any base current to Q4, so it stays cut off.

a and b sometimes have a positive value

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.